Chemical-thermal treatment is a thermal treatment process in which a metallic workpiece is placed in an active medium with a certain temperature, and one or more elements permeate into its surface. Thus, its chemical composition, microstructure and properties are changed. There are many kinds of chemical-thermal treatment methods, and the most common methods are carburizing, nitriding and carbonitriding. The purpose for chemical-thermal treatments in general is to improve the surface wear resistance, fatigue strength, corrosion resistance and high temperature oxidation resistance. The article “Behavior and Mechanism of TiAl Based Alloy Surface Carburization” by Yao Jiang, Yuehui He, et. al., published in Vol. 19, No. 2, Chinese Journal of Materials Research, April 2005, proposed that the high temperature oxidation resistance of TiAl based alloy can be improved through carburizing. Another article, “The Surface Carburizing Treatment Methods of TiAl Based Alloys” by Qiang Xu, Xin-yan Tang and Yang Pu present a similar view. Currently, chemical-thermal treatment processes are mainly used to improve surface properties of relatively dense metallic materials, whereas its application on porous metal materials has not been reported yet.
On the other hand, due to the permeability of porous metal materials, a variety of filter elements made of porous metal materials have been developed. Common porous metal materials are stainless steels, copper and copper alloys, nickel and nickel alloys, titanium and titanium alloys. These kinds of porous metal materials have relatively good machinability but relatively poor corrosion resistance. Another kind of porous metal materials are aluminum-based intermetallic compound porous materials, mainly including TiAl intermetallic compound porous materials, NiAl intermetallic compound porous materials and FeAl intermetallic compound porous materials. These porous metal materials have both excellent machinability and good corrosion resistance. Both the common porous metal materials and the Al-based intermetallic compound porous materials are manufactured by powder metallurgy methods, and in the manufacturing process many factors can affect the final pore diameters of the porous metal materials, such as the average particle size, particle size distribution, particle shape and sintering temperature.
In summary, for now people in this field usually adjust the pore diameters only from the perspective of powder metallurgy processes to fit different filtering requirements. Since adjusting the powder metallurgy processes would easily change the mechanical properties of the materials, feasible solutions can be ascertained usually through a lot of trials, and the range of adjustable pore diameters is limited.